The present invention relates to a method and device for joining ends of material strips made of rubber or rubber-like plastic materials (plastic material having rubber, i.e. elastic, properties) with thin cross-sectional areas, especially thin material strip edges, wherein the end of the material strip which has been cut first is stretched immediately before joining the two ends.
In the manufacture of tires it is often required that material strips with thin material strip edges must be joined on a winding drum to a closed ring. The material strips are cut with a separating cut to the desired length and are placed onto the winding drum such that the ends of the material strips overlap in a defined manner. However, it is necessary that material accumulations within the overlap portion are to be prevented in order to avoid imbalance of the finished tire. The term overlap length refers to the amount of overlap of the two ends of the material strip which, due to the slanted cut (slanted with respect to the face of the material strip) of the two ends, will result upon joining without causing at the joint a material portion thicker than the material strip thickness. According to this definition, an abutting of the two ends for vertical cuts would result in an overlap length of zero while with increasing slant of the ends the overlap length will increase. Furthermore, it is easily understood that the overlap length in cross-sectional areas of greater material thickness is greater than in areas of thinner material thickness.
In practice, this results in the problem that the material strip, as a function of the rubber quality of the unvulcanized rubber mixture, will retract at the cut ends in areas of reduced thickness to a greater or lesser extent so that a uniform joining of the ends is made more difficult. This leads primarily to uncertainty with respect to the durability of the joint (splice) in the areas of reduced thickness.
This effect becomes furthermore more pronounced, when material strips consisting of different parts and mixtures are to be joined as is, for example, necessary for the premanufacture of sidewalls which, in general, are produced as an intermediate product either together with the bead strips or with portions neighboring the bead strips. In such material strips thinner portions are not only present within the edge portions but also within the central portion of the material strip.
The operator who winds the material strips can try to manually stretch the rearward end within the areas of reduced thickness during joining. However, this results in a non-uniform joining and, on the other hand, the main problem is not the rearward end of the material strip but the first cut and first applied end (forward end) of the strip of material.
U.S. Pat. No. 4,470,866 shows a device in which a material strip to be wound onto a drum and to be joined is engaged at its forward end facing the drum by grip elements 23 within the areas of reduced wall thickness and then conveyed to the drum. At the rearward end and within the central portion of the material strip, a braking roller reduces the transporting speed. This results in a forward stretching of the thin edges with the goal of compensating a possible retraction. Disadvantageously, a defined stretching is not possible with such a device because due to the different fixation points, respectively, braking points at the forward and rearward ends of the material strip, a stretched, respectively, tensioned state will result over the entire length of the material strip to be applied to the drum. After removal of the forward clamps, the material strip has the tendency to retract into a tension-free and thus undefined state.
U.S. Pat. No. 5,292,398 discloses a device in which the disadvantages of providing a tensioned state over the entire length of the material strip have been eliminated by providing a small distance between the clamps 29 that fix the central portion of the material strip and the clamps 109 that stretch the outer thin edges of the material strip. However, this device has the disadvantage that the entire stretching is performed at the rearward end of the material strip so that the clamping device itself must remain attached to the material strip for maintaining the tensioned state until the rearward end is applied to the building drum. Thus, the construction of the device requires a plurality of guide elements so that the entire rearward clamping and tensioning apparatus can be moved into a position at the building drum.
Before the next material strip can be placed onto the drum, the clamping and tensioning device must be returned into its initial position. This return action is disadvantageous with respect to manufacturing times.
U.S. Pat. No. 4,804,426 shows a handling device which is essentially comprised of a robot arm and a corresponding support head. The support head 9 is constructed such that central and lateral parts of an oncoming material strip are separately clamped, respectively, secured by vacuum and pulled onto the drum whereby either the forward or the rearward end of the material strip is stretched by an additional forward movement for rotation of the clamping devices that secure the thin lateral edges.
This device has the disadvantage that the clamping devices acting on the thin lateral edges of the material strip at least in part must come into contact with the building drum and must then be laterally removed from the work table. A stretching of the lateral parts is thus possible, however, a precise application of the material strip onto the drum is made more difficult.
In all of the previously known solutions it is a further disadvantage that the known devices comprise clamping devices which act onto the bottom face as well as the top face of the material strip so that the bottom face to be bonded adhesively in further manufacturing steps has already been subjected to mechanical loading instead of being simply guided load-free to the final adhesion step.
It is therefore an object of the present invention to provide a device of the aforementioned kind with which a joint (splice) of great durability can be produced within the areas of reduced material thickness, within edge portions as well as in the central portion of the material strips, with reproducible product quality.